1. Field of the Invention
The present invention relates to a wafer stage of a photolithographic exposure apparatus. More particularly, the present invention relates to a magnetic levitation wafer stage which levitates a wafer to be exposed.
2. Description of the Related Art
Typically, a wafer used for fabricating a semiconductor device is repeatedly and selectively subjected to individual processes such as cleaning, diffusion, photo-resist coating, exposure, development, etching, and ion-implantation processes. These processes are performed by respective apparatuses. There are several types of exposure apparatus for performing the exposure process. Among these apparatus is a stepper. A stepper is used to direct light from a light source through a reticle, and to scan a layer of photo-resist on the surface of the wafer with the light directed through the reticle. The reticle bears a pattern corresponding to that of a circuit pattern or the like. The layer of photo-resist is thus exposed to an image of the pattern of the reticle. The exposed layer of photo-resist is then developed to remove the exposed (or non-exposed) regions of the layer and thereby pattern the layer of photoresist. The underlying layer is then etched using the patterned layer of photo-resist as a mask. Accordingly, a pattern corresponding to that of the reticle is formed on the wafer.
Generally, the wafers are exposed one-by-one in the stepper. To this end, a predetermined number of wafers coated with photo-resist are loaded in a carrier and the carrier is transferred to a loading/unloading station inside the stepper. Then, a robot arm having a blade is extended to insert the blade into the carrier beneath a wafer. Next, the table is moved down so that the wafer is supported by the blade, and the robot arm is retracted to remove the wafer from the carrier. Once the wafer is removed from the carrier, the wafer is transferred from the blade to a horizontally movable member of a transfer device. The horizontally movable member loads the wafer onto a wafer stage of the stepper. The wafer stage of the stepper aligns each die (region) of the wafer with the reticle whereupon the exposure of the die commences, and the wafer stage moves the wafer so that the dies are exposed sequentially.
The wafer stage has a ceramic chuck (table) and a driving device for moving the chuck. The ceramic chuck holds the wafer, and the driving device moves the ceramic chuck in X and Y (orthogonal) directions. A transfer arm unloads the wafer from the ceramic chuck once the exposure of the wafer has been completed.
During the above-described operation, particles are generated from the back of the wafer or elsewhere in a vacuum chamber in which the wafer stage is disposed. The particles accumulate on the ceramic chuck. Such particles may cause defocus, i.e., an inability of the exposure apparatus to properly focus the image of the pattern of the reticle on the wafer. Therefore, after a certain number of wafers have been exposed, an engineer stops the operation of the exposure apparatus whereupon the pressure in the vacuum chamber of the exposure apparatus is returned to atmospheric pressure. The engineer then takes the wafer stage out of the apparatus, and cleans the ceramic chuck to remove any particles on the chuck.
U.S. Pat. No. 5,196,745 discloses a positioning device (a stage) that can prevent defocus errors from occurring. The positioning device disclosed in U.S. Pat. No. 5,196,745 has a table (wafer chuck) that is magnetically suspended so as to be out of contact with a stationary member in which the table is received. Permanent magnets are arrayed in two dimensions on both top surface and bottom surface of the table. The permanent magnets are arranged, in each direction of the array, such that the adjacent poles of the magnets are of opposite polarity. Also, a multiphase coil array corresponding to the permanent magnets is mounted to the stationary member, in which the table is received, for suspending the table and driving the table in horizontal directions.
However, the table of the non-contact type of positioning device disclosed in U.S. Pat. No. 5,196,745 is relatively large and heavy because the table has a plurality of permanent magnets on the top and bottom surfaces thereof. Moreover, the multiphase coil array can only produce a considerably weak magnetic field in a vertical direction. Therefore, it is difficult to devise a working embodiment in which the multiphase coil array suspends such a heavy table satisfactorily.